The invention relates to a method for producing an injection molded article with a desired cellular density distribution, and more particularly, to a method for producing the injection molded article whereby the cellular structure of the injection molded article gradually varies between a cellular structure of high density at the perimeter of the molded article to a cellular structure of relatively low density in the interior of the molded article.
Plastic injection molding machines generally include a mold unit and an injection assembly. The mold unit typically contains a stationary mold section and a movable mold section, whereby the injection molding machine can form a mold cavity by engaging the movable mold section with the stationary mold section. The stationary mold section generally contains a runner system that allows for injection of an injection mixture into the mold cavity. An injection nozzle engages with the stationary mold section runner system thereby allowing injection of the injection material into the mold cavity.
In the injector assembly, the injection mixture is prepared for injection. Maintaining the injection mixture under pressure, and at the desired temperature, controls the viscosity of the injection mixture prior to injection. It is often desired to create a plastic article of a decreased density, as the decreased density reduces the amount of material needed to create the article and it also reduces the weight of the article. It is well known in the art to combine a blowing agent with the injection material to accomplish this decrease in density. Most blowing agents are heat activated and produce bubbles, or foam, within the plastic when the blowing agent activates. Therefore, the blowing agents activate above a specific activation temperature. If the temperature of the blowing agent is maintained below the specific activation temperature the blowing agent does not activate. Additionally, the bubbles produced by the blowing agent will generally be evenly distributed throughout the plastic article after the blowing agent is activated. As the plastic material cools and hardens, these bubbles form a cellular structure throughout the plastic article of a given density.
It is generally preferable that the outer surface of the injection molded article be smooth and uniform. This smooth and uniform outer shell gives the plastic article a more aesthetically pleasing appearance and a wear resistant outer covering. Accordingly, there is a need to create a plastic injection molded article with an inner low density cellular core and a more dense outer skin that is free from the cellular structure created by the blowing agent. U.S. Pat. No. 5,437,823 shows how to make a plastic article with a dense unfoamed outer skin and a foamed interior. This is achieved by injecting the plastic material and blowing agent into the mold cavity under high enough pressure to prevent the blowing agent from forming bubbles. The plastic material is maintained at a high pressure while a skin forms along the walls of the mold cavity. Then the pressure is decreased and the blowing agent is allowed to foam in the interior of the plastic article.
This process, however, generally results in a molded article with a uniform low density core and an abrupt transition to a smooth high density exterior. Injection molded articles molded according to this process are typically not very compressible and do not have a good memory for compressibility, even when molded from relatively pliable thermoplastic materials. In many instances it is desirable to produce an injection molded plastic article with a high density smooth outer cellular structure and a relatively lower density cellular structure at the core, which is compressible and has a memory for compressibility. Consequently, there is a need to produce an injection molded article with a tough wear resistant exterior surface that retains a cushion feel when under a load.
Further, this type of process performed with substantially rigid thermoplastic materials, while reducing the weight of the articles over articles produced without blowing agents, still produces somewhat heavy articles and cannot distribute the weight within the article to produce area of varying density. For example, floats utilizes in a wide variety of uses must be very light weight puncture resistant and often must be weighted such that some portions of the float are lighter than others. Floats are commonly used in toilets, gas tanks, carburetors, cooling towers, humidifiers, ice making equipment, car wash equipment, swimming pool equipment, and a host of other applications that require liquid sensing and/or liquid metering. These floats are typically constructed of copper, steel, nickel, rubber, polystyrene, or urethane foam. Many of these materials, however, are heavy, expensive, difficult to manufacture, and incapable of construction with varying densities.